Recently, magnesium alloys, which are light in weight, have been attracting attention as structural metal materials. However, a typical magnesium alloy, AZ31 (3 mass % Al, 1 mass % Zn, and the balance of Mg), when rolled, has inferior cold workability, and cannot be pressed at lower than about 250° C. While magnesium takes the hcp crystal structure (α phase), magnesium-lithium alloys, which contain lithium, take a mixed phase of the hcp structure and the bcc structure (β phase) at a lithium content of 6 to 10.5 mass %, and a single β phase at a lithium content of 10.5 mass % and higher. As is known widely, slip in the α phase is limited, but the β phase has a number of slip systems. The cold workability of magnesium-lithium alloys improves as the lithium content increases so that the phase changes from the α/β mixed phase to the single β phase. However, since lithium is an electrochemically lower element, increase in the lithium content results in significant deterioration of the corrosion resistance of the alloys. On the other hand, alloys with a higher lithium content, such as LA141 (14 mass % Li, 1 mass % Al, and the balance of Mg), have also been developed. But these alloys are limited in use due to their insufficient corrosion resistance.
Patent Publication 1 teaches that magnesium-lithium alloys with a lithium content of not higher than 10.5 mass % and an iron impurity concentration of not higher than 50 ppm, have excellent corrosion resistance.
Patent Publication 2 teaches that magnesium-lithium alloys containing 6 to 10.5 mass % lithium and 4 to 9 mass % zinc have excellent strength and corrosion resistance at room temperature.
Patent Publication 3 discloses magnesium-lithium alloys containing 6 to 16 mass % lithium, which are suitable for cold-pressing.
Patent Publication 4 teaches that magnesium-lithium alloys having a lithium content of 10.5 to 40 mass % and an average crystal grain size of 3 to 30 μm, have excellent strength and press workability.
Non-Patent Publication 1 discloses effects of addition of Al, Zn, Cu, and Ag to magnesium-lithium alloys with a lithium content of 8 mass % and 13 mass % on their mechanical characteristics or corrosion resistance when subjected to processing or heat treatment.
In the prior art, however, no magnesium-lithium alloy has hitherto been obtained which contains not less than 10.5 mass % Li, is of the single β phase, and has both corrosion resistance and cold workability well balanced. No such single β phase magnesium-lithium alloy is known that has mechanical strength, e.g., a tensile strength of not lower than 150 MPa. For example, Patent Publication 4 discloses magnesium-lithium alloys having excellent strength and press workability, but the tensile strength of the alloys containing not less than 10.5 mass % Li disclosed in Examples is 131 MPa at most.
Patent Publication 4 also discloses a method for producing a magnesium-lithium alloy having excellent strength and press workability, including subjecting a magnesium-lithium alloy raw material ingot to hot rolling, cold rolling, and then heat-treating at 140 to 150° C. to recrystallize.
It is also disclosed that, in this method, the cold rolling at a higher reduction of 30 to 60% provides abetter rolled material than at a lower reduction of 20 to 25%. On the other hand, it is also disclosed that, in the same method, the heat treatment for recrystallization of the magnesium-lithium alloy at over 150° C. results in excess increase in the average crystal grain size of the obtained alloy, failing to obtain the desired effects. Thus, the summary of the teachings of Patent Publication 4 is that cold rolling at a higher reduction is preferred for better rolled materials, whereas the heat treatment for recrystallization should be done at 150° C. at most in order to obtain magnesium-lithium alloys with excellent strength and press workability.
Further, magnesium-lithium alloys as mentioned above are under discussion for use as a material composing the casing parts of various electrical instrument which are expected to be lightweight, such as mobile phones, notebook PCs, video cameras, and digital cameras. For such use, the alloys are required to have a low surface electrical resistivity for ensuring sufficient electromagnetic shielding ability and for providing ground to the substrates. Thus magnesium-lithium alloys with a low surface electrical resistivity are desired.